Tracking the position of an invasive medical instruments is required in many cardiac procedures. For example, U.S. Pat. No. 5,983,126 describes a system and method for catheter location mapping, and related procedures. Three substantially orthogonal alternating signals are applied through the patient, directed substantially toward the area of interest to be mapped, such as patient's heart. A catheter is equipped with at least a measuring electrode, which for cardiac procedures is positioned at various locations either against the patient's heart wall, or within a coronary vein or artery. A voltage is sensed between the catheter tip and a reference electrode, preferably a surface electrode on the patient, which voltage signal has components corresponding to the three orthogonally applied current signals. Three processing channels are used to separate out the three components as x, y and z signals, from which calculations are made for determination of the three-dimensional location of the catheter tip within the body.
As another example, U.S. Patent Application Publication 2013/0267835 describes a system and method for localizing medical instruments during cardiovascular medical procedures is described. One embodiment comprises an electromagnetic field generator; an antenna reference instrument adapted to be introduced into the heart of a subject and including at least one electromagnetic sensor and at least one electrode; at least one roving instrument adapted to be introduced into the thorax cavity of the subject and including at least one electrode; and a control unit configured to determine position coordinates of the antenna reference instrument based on an electromagnetic signal from the electromagnetic field generator sensed by the electromagnetic sensor, measure an electrical-potential difference between the electrode of the antenna reference instrument and the electrode of the roving instrument, and calibrate the measured electrical-potential difference using the determined position coordinates of the antenna reference instrument to determine position coordinates of the roving instrument.
U.S. Pat. No. 7,536,218 describes a position tracking system includes a probe adapted to be introduced into a body cavity of a subject. The probe includes a magnetic field transducer and at least one probe electrodes. A control unit is configured to measure position coordinates of the probe using the magnetic field transducer. The control unit also measures an impedance between the at least one probe electrodes and one or more points on a body surface of the subject. Using the measured position coordinates, the control unit calibrates the measured impedance.
U.S. Pat. No. 8,456,182 describes a method that includes positioning surface-electrodes in galvanic contact with a body of a patient and positioning a calibration-tool, having a calibration-electrode, in a plurality of regions in the body. The method further includes tracking the calibration-tool at different positions in each of the regions using a magnetic based location-measuring system, and for each region, generating a respective set of calibration-currents between the surface-electrodes and the mapping-electrode at the different positions in the region. A respective relation is derived for each region between the respective set of the calibration-currents and the different magnetically tracked positions, and is used in determining the location of an investigation-tool in response to the different respective relations and investigation-tool-currents.